98 DIFFUSION OF GASES 261 



For by substitution we obtain for the value of the coefficient 

 of diffusion 



D = Jr + N 



wherein the magnitudes F have the meanings 



4- 



given before. This value is by no means constant ; for it 

 depends not only on the temperature by reason of Oj and O 2 

 and on the pressure of the mixture by reason of N, but 

 it varies also with N l and N 2 , and thus with the ratio in 

 which the two gases have already mixed with each other. 



The dependence of the coefficient of diffusion on the 

 temperature and pressure of the gaseous mixture can raise 

 no doubt. The coefficient, however, is to be looked on as a 

 constant in this respect, that it has the same value every- 

 where throughout the space filled by the gas in an experiment 

 on diffusion, provided that the temperature is kept constant ; 

 for the pressure equalises itself everywhere if the diffusion 

 proceeds without a separating partition. 



It is also in agreement with experiment if the coefficient 

 of diffusion varies in value with the temperature and total 

 pressure of the gaseous mixture. Loschmidt observed 

 that the value of the coefficient was found to be the greater 

 the higher the temperature in the experiment ; and this is 

 shown by the formula too. From the formula also it follows 

 that the value of the coefficient of diffusion alters with the 

 total pressure of the mixture in such wise that D is inversely 

 proportional to this pressure, because the number N of the 

 molecules contained in unit volume varies directly as the pres- 

 sure. This too is in perfect agreement with Loschmidt's 

 observations. 



But our theoretical formula exhibits still another varia- 

 tion of the coefficient of diffusion, which we should not 

 a priori expect. The value of D is dependent also on the 

 values of N l and N 2 , and these numbers, which express how 

 many molecules of the one or other kind there are in unit 

 volume of the mixture, alter on their side with x, as in 95, 

 and therefore with the position in the tube. The coefficient 



